The present invention relates to a method of adjusting a frequency response of a ladder-type electric filter to be used for a filter circuit in a portable and mobile radiotelephony set or an automobile telephone set comprising series resonators and parallel resonators contained in a case to form a unit filter circuit to be mounted on a print circuit board as a component of a filter circuit having a given circuit configuration.
It is known that such a ladder-type electric filter comprises series resonators with electrodes one of which is connected to an input terminal and parallel resonators with electrodes one of which is grounded, the other electrode of each of the series and parallel resonators being connected to an output terminal. Known conventional ladder-type electric filters include those comprising square series and parallel resonators with sides that are approximately 5 mm long and designed to operate at a center frequency of 455 KHz, utilizing an contour vibration mode and those comprising circular series and parallel resonators having an diameter of approximately 5 mm to utilize a radial vibration mode for operation.
Recently, on the other hand, with the increasing demand for down-sized and lightweight wireless telecommunications equipment, the ladder-type electric filters are required to reduce their height to as small as 0.01 mm. Such a rigorous requirement, however, cannot be met by conventional filters comprising resonators that are arranged to a multilayer structure and put in a case.
In an attempt to provide a breakthrough to the above problem, there has been proposed a ladder-type electric filter which comprises a pair of square parallel resonators superimposed or vertically arranged one on the other to utilize a contour vibration mode, each parallel resonator being provided with electrodes on its upper and lower surface, the electrodes arranged on the interface being connected to a grounding terminal plate, and a pair of strip-shaped series resonators juxtaposed or horizontally,arranged side by side on the upper parallel resonator with an insulating plate interposed therebetween to utilize a longitudinal vibration mode, each series resonator having electrodes on its upper and lower surface, the electrodes arranged on the respective lower surfaces of the series resonators being connected to the corresponding lower surface electrode of the lower parallel resonator by way of a terminal plate, the upper surface electrode of one of the series resonators being connected to an input terminal plate, the upper surface electrode of the other series resonator being connected to the upper surface electrode of the upper parallel resonator by way of an output terminal plate, said components being contained in a case, the input, output and grounding terminal plates being provided with respective connector legs projecting out of the case for connection with external electric circuits. A filter having a configuration as described above has a reduced overall height when compared with conventional filters, because of the horizontal arrangement of the series resonator.
Japanese Utility Model Application No. 3-33539 also discloses a ladder-type electric filter with a reduced overall height comprising a pair of strip-shaped series resonators respectively arranged by a pair of parallel resonators.
In either case, it is appreciated that a pair of strip-shaped series resonators are the key to the reduction of the height of a ladder-type electric filter.
As described above, such a ladder-type electric filter is required to have an operating frequency that strictly adheres to a given value and the resonance frequencies of its strip-shaped series resonators need to be regulated in order to accommodate themselves to any discrepancy in the operating ,frequency of the filter that may be given rise to by inaccurate molding and other causes. A conventional technique for such regulation of frequency consists in cutting either of series resonators S1 and S2 along a shorter edge .alpha. as illustrated in FIG. 1 of the accompanying drawings in order to reduce its length and hence increase its resonance frequency.
FIG. 2 schematically illustrates the relationship between the length of a series resonator and its resonance frequency. As seen in FIG. 2, the resonance frequency of the series resonator is directly proportional to its length and their relationship can be represented substantially by a straight line. Thus, the resonance frequency of the series resonator can be regulated by modifying its length.
On the other hand, however, if the length of the series resonator is modified, it can be displaced, even if slightly, from its proper position in the case when it is inserted into the case so that its center (where the node of vibration is located) p2 may be misaligned with the location p1 of the corresponding raised contact point of the contiguous terminal plate to increase the insertion loss. This is a problem that adversely affects the stabilized operation of a parallel resonator.
It is therefore an object of the present invention to provide a method of adjusting the frequency of a ladder-type electric filter which is capable of overcoming the disadvantages involved in the proposed frequency adjusting methods for any conventional ladder-type electric filter.
Another object of the invention is to provide a ladder-type electric filter that is free from the above identified problem.
According to the present invention, the above first object is achieved by providing a method of adjusting the frequency of a ladder-type electric filter comprising parallel and series resonators arranged and connected to form a given fundamental filter circuit configuration, said series resonators being of strip-shaped, wherein the strip-shaped series resonators are partially obliquely cut at least one of corner portions of either of the resonators without changing its length to regulate the resonance frequency of the resonators.
After a series of experiments, it has been found that the resonance frequency of the strip-shaped series resonators is increased by beveling the corner portions of the respective resonator and the increase in the frequency is proportional to the extent of beveling.
By beveling the corner portions of the strip-shaped resonators, the length thereof is not affected at all and hence it can always be inserted into the corresponding chamber of the case without the risk of misalignment so that the corresponding raised contact point of the contiguous terminal plate securely touches the center of the resonators, which provides the node of vibration, ensuring an effective filtering operation.